1. Field of Invention
The present invention is directed to an expandable centralizer for centering an expandable casing string within a borehole. More specifically, the present invention is directed to an apparatus for and a method of deploying an expandable centralizer after it is installed on an expandable casing string and run into a borehole to generally center an expandable casing string at a desired position within the borehole.
2. Background of Related Art
Centralizers are commonly secured at intervals along a pipe string, such as a casing string, to provide stand-off between the exterior wall of the casing string and the wall of a borehole in which the casing string is installed. The centralizers generally include a plurality of angularly distributed ribs that project radially outwardly from axis of the casing string to provide “stand-off” between the wall of the borehole and the casing string. Centralizers ideally center the casing string within the borehole to provide a generally uniform annulus between the casing string and the wall of the borehole. Positioning of the casing string within a borehole promotes uniform distribution of cement slurry around the casing string to form a protective liner that reinforces the casing string, isolates the casing string from corrosive formation fluids, and prevents unwanted fluid flow between penetrated geologic formations.
A bow-spring centralizer employs flexible bow-springs as the ribs to provide stand-off. Bow-spring centralizers typically include a pair of axially-spaced and generally aligned collars that are coupled one to the other by multiple curved bow-springs. The bow-springs are predisposed to bow outwardly away from the axis of the centralizer to engage the borehole and to center a casing string received through the aligned bores of the spaced-apart collars. Configured in this manner, the bow-springs provide stand-off from the borehole, but flex inwardly to cooperate with borehole obstructions, such as tight spots or protrusions into the borehole, as the casing string is installed into the desired interval of the borehole. Bow springs are generally cold formed, heat treated and otherwise forged, shaped or adapted to collapse to lie generally along the length of the casing and to spring back to substantially their original bowed shape to maintain the desired stand-off between the casing string and the borehole.
Casings strings having centralizers installed thereon are generally run into boreholes using force to overcome the resistance of the bow springs. A deployed bow spring centralizer generally engages the interior wall of a previously installed casing string, or the wall of the borehole, through multiple or all bow springs at any given position in a borehole. If bow spring centralizers are secured at generally uniform intervals along the length of a casing string, the cumulative amount of resistance due to the frictional drag on each bow spring can be large. Each deployed bow spring contributes to the overall resistance to movement of the casing string within the borehole, such as when the casing string is being positioned in the borehole for cementing. Also, the force necessary to collapse the bow springs of a centralizer to cause them to lie against the casing on which the centralizer is installed and to thereby pass a borehole restriction may be many times the resistance force that must be overcome to move the collapsed centralizer through the borehole restriction. Accordingly, it may be preferred to run the centralizers into the surface end of the borehole in a collapsed configuration if the centralizers can be reliably deployed after the casing string is positioned at the targeted borehole interval.
Some centralizers have been adapted to withstand expansion, along with the casing string, and generally provide stand-off between the expanded casing string and the borehole. Other centralizers have been adapted for being run into the borehole after being installed on a casing string, but restrained in a collapsed configuration to minimize the resistance to movement of the casing string through the borehole and to the targeted interval of the borehole. However, the known methods of deploying a collapsed and restrained bow spring centralizer within the targeted interval of a borehole are often complicated or involve the use of strong chemical agents. For example, in U.S. Pat. No. 5,261,488, a method of deploying a restrained centralizer by dissolving or degrading a titanium band using hydrofluoric acid is disclosed. These types of chemical agents may damage other structures or present a hazard to personnel that ship or handle the agents at the surface.
What is needed is a method for installing an expandable centralizer on a casing string, restraining the expandable centralizer in a collapsed configuration, installing the casing string in the targeted interval of a borehole and then deploying the expandable centralizer to provide stand-off between the casing string and the borehole. What is needed is a method of restraining an expandable centralizer in its collapsed configuration to facilitate installation of the casing string in a targeted interval in a borehole, and a method of deploying the centralizer (after the casing string is run into the borehole to the targeted interval) that does not require the introduction of strong chemical agents into the borehole, complicated mechanisms or difficult manipulations of the casing string. What is needed is an expandable centralizer that reliably deploys simultaneously upon expansion. What is needed is an expandable centralizer that is controllably deployable within the borehole at or near the position in which the centralizer is cemented within the annulus. What is needed is a centralizer that is simultaneously expandable and deployable to provide stand-off between a casing and a borehole.